1. Field of the Invention
This invention relates to a mutual scan method for a touch panel, in particular to a mutual scan method for increasing mutual sensing frame rates and preventing interference resulting from noises.
2. Description of Related Art
Scan methods for a capacitive touch panel include a mutual scan approach and a self scan approach. The two approaches are able to recognize touch objects respectively based on information of mutual sensing frames and self sensing frames.
The touch objects are recognized according to sensing capacitance variation values at the positions where the touch objects are present. Because the capacitive touch panel recognizes the touch objects based on the capacitance variation values of column sensing lines and row sensing lines, the touch panel is easily interfered by environment noises so that the accuracy for identifying touch objects is degraded. The environment noises may be AC noise, LCM noise, etc.
After the touch panel performs the self scan approach, a waveform of a sensing signal produced by sensing lines X1˜XN in one axis of the touch panel is shown in FIG. 7A. The sensing signal comprises a low sensitivity touch signal Sstylus (produced by a stylus 20 with a small diameter), a noise sensing signal Snoise (induced by AC noise 11 or LCM noise 12) and a high sensitivity touch signal Sfinger (produced by a finger 30 or a stylus with a large diameter). With reference to FIG. 7B, after the touch panel executes the mutual scan approach, a mutual sensing frame 10 also has information of the AC noise Snoise, the high sensitivity touch signal Sfinger and the low sensitivity touch signal Sstylus as shown in FIG. 7A.
With reference to FIG. 7A, a capacitance sensing value of the high sensitivity touch signal Sfinger is larger than those of the low sensitivity touch signal Sstylus and the noise sensing signal Snoise. The touch object with the high sensitivity touch signal Sfinger can be recognized easily. By setting a sensing threshold dV1, the presence of the touch object is confirmed if the sensing value is larger than the sensing threshold dV1. However, when the capacitive touch panel is touched by the stylus 20, the low sensitivity touch signal Sstylus is generated and smaller than the sensing threshold dV1. The touch object of the stylus 20 cannot be recognized. If the original sensing threshold dV1 is decreased to a new sensing threshold dV2, the noise sensing signal Snoise will be recognized as a touch object incorrectly according to the new sensing threshold dV2.